Brillouin-scattering study of the liquid-glass transition in supercooled aqueous lithium chloride solutions: Generalized hydrodynamics and mode-coupling analyses

Effect of the Hydrocarbon Chain Disorder in Phosphatidylcholine Bilayers on Gigahertz Sound Velocity

Suspensions of multilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and hydrated and dehydrated aligned multilamellar samples of DMPC were studied by Brillouin spectroscopy in the temperature range from 90 to 333 K. The sound velocity of the longitudinal acoustic wave was evaluated from the Brillouin spectra. It was found that phase transition, hydration state, and planar or vesicular form of bilayers affect the gigahertz sound velocity. Usually, the temperature dependence of the sound velocity is weak in solid substances. Amazingly, the sound velocity of hydrated DMPC samples showed significant temperature-induced changes of up to 1.8 times, even within the solid-like gel phase. We explained this effect by temperature-induced excitations of the disordered conformational states of the hydrocarbon chains as well as anharmonic effects. In addition, the relevance of the gigahertz sound velocity to the description of subterahertz Raman features was demonstrated.

Ultraviolet and visible Brillouin scattering study of viscous relaxation in 3-methylpentane down to the glass transition

Brillouin light scattering spectra from transverse and longitudinal acoustic waves in liquid and supercooled 3-methylpentane have been collected from room temperature down to 80 K, just above the glass transition. Spectra at different wave vectors have been obtained using 532 nm and 266 nm excitation. We found evidence of a shear relaxation with a characteristic time of 100 s at the glass transition which only partly accounts for the relaxation observed in the propagation and attenuation of the longitudinal modes. The inclusion of a relaxing bulk viscosity contribution with a relaxation time of the order of 10(2) ns at the glass transition is found to adequately reproduce the experimental data including transient grating data at a much lower frequency. A consistent picture of relaxed shear and bulk moduli as a function of temperature is derived. These two quantities are found to be related by a linear relation suggesting that a Cauchy-like relation holds also above the glass transition.

Water dynamics in a lithium chloride aqueous solution probed by Brillouin neutron and x-ray scattering

We studied the collective excitations in an aqueous solution of lithium chloride over the temperature range of 270-205 K using neutron and x-ray Brillouin scattering. Both neutron and x-ray experiments revealed the presence of low- and high-frequency excitations, similar to the low- and high-frequency excitations in pure water. These two excitations were detectable through the entire temperature range of the experiment, at all probed values of the scattering momentum transfer (0.2 Å(-1) < Q < 1.8 Å(-1)). A wider temperature range was investigated using elastic intensity neutron and x-ray scans. Clear evidence of the crossover in the dynamics of the water molecules in the solution was observed in the single-particle relaxational dynamics on the µeV (nanosecond) time scale, but not in the collective dynamics on the meV (picosecond) time scale.

Brillouin spectroscopy studies of two-component polymerizable liquid system: 2,2-bis[4-(2-hydroxymethacryloxypropoxy)phenyl]propane/benzyl methacrylate

Brillouin spectroscopy was used to investigate viscoelastic properties of a two-component system consisting of a high viscosity liquid (HVL) and a low viscosity liquid (LVL), both able to polymerize. The model liquids were: 2,2-bis[4-(2-hydroxymethacryloxypropoxy)phenyl]propane (abbreviated as bis-GMA, HVL) and benzyl methacrylate (BzMA, LVL). The viscosity of the system was regulated by changing the monomer ratio. Hypersonic velocity and attenuation coefficient were investigated in a temperature range covering viscoelastic relaxation process. The dependence of the longitudinal viscosity on the system composition was determined. Additionally, the Brillouin studies were accompanied by some supplementary experimental methods, like low frequency shear viscosity measurements and observations of phase transitions by differential scanning calorimetry (DSC). The investigated monomer mixtures were then polymerized in a light-induced process and the polymerization kinetic curves were measured to find the possible correlation between the viscoelastic properties of the monomer mixture (as observed by Brillouin spectroscopy) and the polymerization course.

The low frequency phonons dynamics in supercooled LiCl, 6 H2O

We report the results of a series of ultrasound, Brillouin scattering, and optical heterodyne detected transient grating experiments performed on a LiCl, 6H(2)O solution from room temperature down to the vicinity of its liquid-glass transition, T(g) approximately 138 K. Down to T approximately 215 K, the supercooled liquid has a behavior similar to what is expected for supercooled water: its zero frequency sound velocity, C(0), continuously decreases while the corresponding infinite frequency velocity, C(infinity), sharply increases, reflecting the increasing importance of H bonding when temperature is lowered. Below 215 K, specific aspects of the solution, presumably related to the role of the Li(+) and Cl(-) ions, modify the thermal behavior of C(0), while a beta relaxation process also appears and couples to the sound propagation. The origin of those two effects is briefly discussed.

Supercooling of aqueous NaCl and KCl solutions under acoustic levitation

The supercooling capability of aqueous NaCl and KCl solutions is investigated at containerless state by using acoustic levitation method. The supercooling of water is obviously enhanced by the alkali metal ions and increases linearly with the augmentation of concentrations. Furthermore, the supercooling depends on the nature of ions and is 2-3 K larger for NaCl solution than that for KCl solution in the present concentration range: Molecular dynamics simulations are performed to reveal the intrinsic correlation between supercoolability and microstructure. The translational and orientational order parameters are applied to quantitatively demonstrate the effect of ionic concentration on the hydrogen-bond network and ice melting point. The disrupted hydrogen-bond structure determines essentially the concentration dependence of supercooling. On the other hand, the introduced acoustic pressure suppresses the increase of supercooling by promoting the growth and coalescence of microbubbles, the effective nucleation catalysts, in water. However, the dissolved ions can weaken this effect, and moreover the degree varies with the ion type. This results in the different supercoolability for NaCl and KCl solutions under the acoustic levitation conditions.

Depolarized light scattering versus optical Kerr effect spectroscopy of supercooled liquids: Comparative analysis

Recently, heterodyne-detected optical Kerr effect (HD-OKE) spectroscopy was used to study dynamics of supercooled molecular liquids. The studies revealed an apparently new physical phenomenon that had not been reported before from the related depolarized light scattering (DLS), namely, an intermediate power law (nearly logarithmic decay) of the response functions [H. Cang et al., J. Chem. Phys. 118, 2800 (2003)]. Conceptually, HD-OKE and DLS data reflect optical anisotropy fluctuations mainly due to molecular reorientation dynamics in time and frequency domains, respectively. The above-mentioned effects are revealed in the mesoscopic range less, similar1 GHz ( greater, similar100 ps), where no direct comparison of the techniques was reported. In this Communication, we attempt such a comparison of exemplifying HD-OKE literature data of the glass-forming salol (phenyl salicylate), benzophenone, and liquid-crystal forming 4-cyano-4(')-pentylbiphenyl with DLS data of the same systems that we measured down to ca. 200 MHz by a combined tandem Fabry-Perot interferometer plus tandem-grating-monochromator technique. Generally, we find a satisfactory agreement, albeit in some cases with subtle differences at frequencies greater, similar10 GHz. We conclude that, in the mesoscopic dynamic range, HD-OKE and DLS studies provide consistent and comparable information, and therefore their conclusions must agree. We argue that the intermediate power law of HD-OKE is in essence a manifestation of the excess wing of the corresponding frequency-domain data, known long since from broadband dielectric spectroscopy and anticipated from DLS studies of supercooled liquids.